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<div class="document" id="extending-embedding-faq">
<h1 class="title">Extending/Embedding FAQ</h1>
<table class="docinfo" frame="void" rules="none">
<col class="docinfo-name" />
<col class="docinfo-content" />
<tbody valign="top">
<tr><th class="docinfo-name">Date:</th>
<td>2004-04-08</td></tr>
<tr><th class="docinfo-name">Version:</th>
<td>7294</td></tr>
<tr class="field"><th class="docinfo-name">Web site:</th><td class="field-body"><a class="reference external" href="http://www.python.org/">http://www.python.org/</a></td>
</tr>
</tbody>
</table>
<div class="contents topic" id="contents">
<p class="topic-title first">Contents</p>
<ul class="auto-toc simple">
<li><a class="reference internal" href="#can-i-create-my-own-functions-in-c" id="id7">1&nbsp;&nbsp;&nbsp;Can I create my own functions in C?</a></li>
<li><a class="reference internal" href="#id1" id="id8">2&nbsp;&nbsp;&nbsp;Can I create my own functions in C++?</a></li>
<li><a class="reference internal" href="#writing-c-is-hard-are-there-any-alternatives" id="id9">3&nbsp;&nbsp;&nbsp;Writing C is hard; are there any alternatives?</a></li>
<li><a class="reference internal" href="#how-can-i-execute-arbitrary-python-statements-from-c" id="id10">4&nbsp;&nbsp;&nbsp;How can I execute arbitrary Python statements from C?</a></li>
<li><a class="reference internal" href="#how-can-i-evaluate-an-arbitrary-python-expression-from-c" id="id11">5&nbsp;&nbsp;&nbsp;How can I evaluate an arbitrary Python expression from C?</a></li>
<li><a class="reference internal" href="#how-do-i-extract-c-values-from-a-python-object" id="id12">6&nbsp;&nbsp;&nbsp;How do I extract C values from a Python object?</a></li>
<li><a class="reference internal" href="#how-do-i-use-py-buildvalue-to-create-a-tuple-of-arbitrary-length" id="id13">7&nbsp;&nbsp;&nbsp;How do I use Py_BuildValue() to create a tuple of arbitrary length?</a></li>
<li><a class="reference internal" href="#how-do-i-call-an-object-s-method-from-c" id="id14">8&nbsp;&nbsp;&nbsp;How do I call an object's method from C?</a></li>
<li><a class="reference internal" href="#how-do-i-catch-the-output-from-pyerr-print-or-anything-that-prints-to-stdout-stderr" id="id15">9&nbsp;&nbsp;&nbsp;How do I catch the output from PyErr_Print() (or anything that prints to stdout/stderr)?</a></li>
<li><a class="reference internal" href="#how-do-i-access-a-module-written-in-python-from-c" id="id16">10&nbsp;&nbsp;&nbsp;How do I access a module written in Python from C?</a></li>
<li><a class="reference internal" href="#how-do-i-interface-to-c-objects-from-python" id="id17">11&nbsp;&nbsp;&nbsp;How do I interface to C++ objects from Python?</a></li>
<li><a class="reference internal" href="#i-added-a-module-using-the-setup-file-and-the-make-fails-why" id="id18">12&nbsp;&nbsp;&nbsp;I added a module using the Setup file and the make fails; why?</a></li>
<li><a class="reference internal" href="#how-do-i-debug-an-extension" id="id19">13&nbsp;&nbsp;&nbsp;How do I debug an extension?</a></li>
<li><a class="reference internal" href="#i-want-to-compile-a-python-module-on-my-linux-system-but-some-files-are-missing-why" id="id20">14&nbsp;&nbsp;&nbsp;I want to compile a Python module on my Linux system, but some files are missing. Why?</a></li>
<li><a class="reference internal" href="#what-does-systemerror-pyimport-fixupextension-module-yourmodule-not-loaded-mean" id="id21">15&nbsp;&nbsp;&nbsp;What does &quot;SystemError: _PyImport_FixupExtension: module yourmodule not loaded&quot; mean?</a></li>
<li><a class="reference internal" href="#how-do-i-tell-incomplete-input-from-invalid-input" id="id22">16&nbsp;&nbsp;&nbsp;How do I tell &quot;incomplete input&quot; from &quot;invalid input&quot;?</a></li>
<li><a class="reference internal" href="#how-do-i-find-undefined-g-symbols-builtin-new-or-pure-virtual" id="id23">17&nbsp;&nbsp;&nbsp;How do I find undefined g++ symbols __builtin_new or __pure_virtual?</a></li>
<li><a class="reference internal" href="#can-i-create-an-object-class-with-some-methods-implemented-in-c-and-others-in-python-e-g-through-inheritance" id="id24">18&nbsp;&nbsp;&nbsp;Can I create an object class with some methods implemented in C and others in Python (e.g. through inheritance)?</a></li>
<li><a class="reference internal" href="#when-importing-module-x-why-do-i-get-undefined-symbol-pyunicodeucs2" id="id25">19&nbsp;&nbsp;&nbsp;When importing module X, why do I get &quot;undefined symbol: PyUnicodeUCS2*&quot;?</a></li>
</ul>
</div>
<div class="section" id="can-i-create-my-own-functions-in-c">
<h1><a class="toc-backref" href="#id7">1&nbsp;&nbsp;&nbsp;Can I create my own functions in C?</a></h1>
<p>Yes, you can create built-in modules containing functions,
variables, exceptions and even new types in C.  This is explained in
the document &quot;Extending and Embedding the Python Interpreter&quot; (<a class="reference external" href="http://docs.python.org/ext/ext.html">http://docs.python.org/ext/ext.html</a>).</p>
<p>Most intermediate or advanced Python books will also
cover this topic.</p>
</div>
<div class="section" id="id1">
<h1><a class="toc-backref" href="#id8">2&nbsp;&nbsp;&nbsp;Can I create my own functions in C++?</a></h1>
<p>Yes, using the C compatibility features found in C++.
Place <tt class="docutils literal">extern &quot;C&quot; { ... }</tt> around the Python include files and put
<tt class="docutils literal">extern &quot;C&quot;</tt> before each function that is going to be called by the
Python interpreter.  Global or static C++ objects with constructors
are probably not a good idea.</p>
</div>
<div class="section" id="writing-c-is-hard-are-there-any-alternatives">
<h1><a class="toc-backref" href="#id9">3&nbsp;&nbsp;&nbsp;Writing C is hard; are there any alternatives?</a></h1>
<p>There are a number of alternatives to writing your own C extensions,
depending on what you're trying to do.</p>
<p>If you need more speed, <a class="reference external" href="http://psyco.sourceforge.net/">Psyco</a> generates x86 assembly code
from Python bytecode.  You can use Psyco to compile the most
time-critical functions in your code, and gain a significant
improvement with very little effort, as long as you're running on a
machine with an x86-compatible processor.</p>
<p><a class="reference external" href="http://www.cosc.canterbury.ac.nz/~greg/python/Pyrex/">Pyrex</a> is a compiler that accepts a slightly modified form of Python
and generates the corresponding C code.  Pyrex makes it possible to write
an extension without having to learn Python's C API.</p>
<p>If you need to interface to some C library for which no Python
extension currently exists, you can try wrapping the library's data
types and functions with a tool such as <a class="reference external" href="http://www.swig.org">SWIG</a>.
For C++ libraries, you can look at <a class="reference external" href="http://www.riverbankcomputing.co.uk/sip/">SIP</a>, <a class="reference external" href="http://cxx.sourceforge.net/">CXX</a>, <a class="reference external" href="http://www.boost.org/libs/python/doc/index.html">Boost</a>, or <a class="reference external" href="http://www.scipy.org/site_content/weave">Weave</a>.</p>
</div>
<div class="section" id="how-can-i-execute-arbitrary-python-statements-from-c">
<h1><a class="toc-backref" href="#id10">4&nbsp;&nbsp;&nbsp;How can I execute arbitrary Python statements from C?</a></h1>
<p>The highest-level function to do this is <tt class="docutils literal">PyRun_SimpleString()</tt> which takes
a single string argument to be executed in the context of the module
<tt class="docutils literal">__main__</tt> and returns 0 for success and -1 when an exception occurred
(including <tt class="docutils literal">SyntaxError</tt>).  If you want more control, use <tt class="docutils literal">PyRun_String()</tt>;
see the source for <tt class="docutils literal">PyRun_SimpleString()</tt> in Python/pythonrun.c.</p>
</div>
<div class="section" id="how-can-i-evaluate-an-arbitrary-python-expression-from-c">
<h1><a class="toc-backref" href="#id11">5&nbsp;&nbsp;&nbsp;How can I evaluate an arbitrary Python expression from C?</a></h1>
<p>Call the function <tt class="docutils literal">PyRun_String()</tt> from the previous question with the
start symbol <tt class="docutils literal">Py_eval_input</tt>; it
parses an expression, evaluates it and returns its value.</p>
</div>
<div class="section" id="how-do-i-extract-c-values-from-a-python-object">
<h1><a class="toc-backref" href="#id12">6&nbsp;&nbsp;&nbsp;How do I extract C values from a Python object?</a></h1>
<p>That depends on the object's type.  If it's a tuple,
<tt class="docutils literal">PyTupleSize(o)</tt> returns its length and <tt class="docutils literal">PyTuple_GetItem(o, i)</tt>
returns its i'th item.  Lists have similar functions, <tt class="docutils literal">PyListSize(o)</tt>
and <tt class="docutils literal">PyList_GetItem(o, i)</tt>.</p>
<p>For strings, <tt class="docutils literal">PyString_Size(o)</tt> returns
its length and <tt class="docutils literal">PyString_AsString(o)</tt> a pointer to its value.
Note that Python strings may contain null bytes so C's <tt class="docutils literal">strlen()</tt>
should not be used.</p>
<p>To test the type of an object, first make sure
it isn't NULL, and then use <tt class="docutils literal">PyString_Check(o)</tt>, <tt class="docutils literal">PyTuple_Check(o)</tt>,
<tt class="docutils literal">PyList_Check(o)</tt>, etc.</p>
<p>There is also a high-level API to Python objects which is
provided by the so-called 'abstract' interface -- read
<tt class="docutils literal">Include/abstract.h</tt> for further details.  It allows
interfacing with any kind of Python sequence
using calls like <tt class="docutils literal">PySequence_Length()</tt>, <tt class="docutils literal">PySequence_GetItem()</tt>, etc.)
as well as many other useful protocols.</p>
</div>
<div class="section" id="how-do-i-use-py-buildvalue-to-create-a-tuple-of-arbitrary-length">
<h1><a class="toc-backref" href="#id13">7&nbsp;&nbsp;&nbsp;How do I use Py_BuildValue() to create a tuple of arbitrary length?</a></h1>
<p>You can't.  Use <tt class="docutils literal">t = PyTuple_New(n)</tt> instead, and fill it with
objects using <tt class="docutils literal">PyTuple_SetItem(t, i, o)</tt> -- note that this &quot;eats&quot; a
reference count of <tt class="docutils literal">o</tt>, so you have to <tt class="docutils literal">Py_INCREF</tt> it.
Lists have similar functions <tt class="docutils literal">PyList_New(n)</tt> and
<tt class="docutils literal">PyList_SetItem(l, i, o)</tt>.  Note that you <em>must</em> set all the tuple items to
some value before you pass the tuple to Python code --
<tt class="docutils literal">PyTuple_New(n)</tt> initializes them to NULL, which isn't a valid Python
value.</p>
</div>
<div class="section" id="how-do-i-call-an-object-s-method-from-c">
<h1><a class="toc-backref" href="#id14">8&nbsp;&nbsp;&nbsp;How do I call an object's method from C?</a></h1>
<p>The <tt class="docutils literal">PyObject_CallMethod()</tt> function can be used to call an arbitrary
method of an object.  The parameters are the object, the name of the
method to call, a format string like that used with <tt class="docutils literal">Py_BuildValue()</tt>, and the argument values:</p>
<pre class="literal-block">
PyObject *
PyObject_CallMethod(PyObject *object, char *method_name,
                    char *arg_format, ...);
</pre>
<p>This works for any object that has methods -- whether built-in or
user-defined.  You are responsible for eventually <tt class="docutils literal">Py_DECREF</tt>'ing the
return value.</p>
<p>To call, e.g., a file object's &quot;seek&quot; method with arguments 10, 0
(assuming the file object pointer is &quot;f&quot;):</p>
<pre class="literal-block">
res = PyObject_CallMethod(f, &quot;seek&quot;, &quot;(ii)&quot;, 10, 0);
if (res == NULL) {
        ... an exception occurred ...
}
else {
        Py_DECREF(res);
}
</pre>
<p>Note that since <tt class="docutils literal">PyObject_CallObject()</tt> <em>always</em> wants a tuple for the
argument list, to call a function without arguments, pass &quot;()&quot; for the
format, and to call a function with one argument, surround the argument
in parentheses, e.g. &quot;(i)&quot;.</p>
</div>
<div class="section" id="how-do-i-catch-the-output-from-pyerr-print-or-anything-that-prints-to-stdout-stderr">
<h1><a class="toc-backref" href="#id15">9&nbsp;&nbsp;&nbsp;How do I catch the output from PyErr_Print() (or anything that prints to stdout/stderr)?</a></h1>
<p>In Python code, define an object that supports the <tt class="docutils literal">write()</tt> method.
Assign this object to <tt class="docutils literal">sys.stdout</tt> and <tt class="docutils literal">sys.stderr</tt>.
Call print_error, or just allow the standard traceback mechanism to
work. Then, the output will go wherever your <tt class="docutils literal">write()</tt> method sends it.</p>
<p>The easiest way to do this is to use the StringIO class in the standard
library.</p>
<p>Sample code and use for catching stdout:</p>
<pre class="literal-block">
&gt;&gt;&gt; class StdoutCatcher:
...     def __init__(self):
...         self.data = ''
...     def write(self, stuff):
...         self.data = self.data + stuff
...
&gt;&gt;&gt; import sys
&gt;&gt;&gt; sys.stdout = StdoutCatcher()
&gt;&gt;&gt; print 'foo'
&gt;&gt;&gt; print 'hello world!'
&gt;&gt;&gt; sys.stderr.write(sys.stdout.data)
foo
hello world!
</pre>
</div>
<div class="section" id="how-do-i-access-a-module-written-in-python-from-c">
<h1><a class="toc-backref" href="#id16">10&nbsp;&nbsp;&nbsp;How do I access a module written in Python from C?</a></h1>
<p>You can get a pointer to the module object as follows:</p>
<pre class="literal-block">
module = PyImport_ImportModule(&quot;&lt;modulename&gt;&quot;);
</pre>
<p>If the module hasn't been imported yet (i.e. it is not yet present in
<tt class="docutils literal">sys.modules</tt>), this initializes the module; otherwise it simply returns
the value of <tt class="docutils literal"><span class="pre">sys.modules[&quot;&lt;modulename&gt;&quot;]</span></tt>.  Note that it doesn't enter
the module into any namespace -- it only ensures it has been
initialized and is stored in <tt class="docutils literal">sys.modules</tt>.</p>
<p>You can then access the module's attributes (i.e. any name defined in
the module) as follows:</p>
<pre class="literal-block">
attr = PyObject_GetAttrString(module, &quot;&lt;attrname&gt;&quot;);
</pre>
<p>Calling <tt class="docutils literal">PyObject_SetAttrString()</tt> to assign to variables in the module also works.</p>
</div>
<div class="section" id="how-do-i-interface-to-c-objects-from-python">
<h1><a class="toc-backref" href="#id17">11&nbsp;&nbsp;&nbsp;How do I interface to C++ objects from Python?</a></h1>
<p>Depending on your requirements, there are many approaches.  To do
this manually, begin by reading <a class="reference external" href="http://docs.python.org/ext/ext.html">the &quot;Extending and Embedding&quot; document</a>.  Realize
that for the Python run-time system, there isn't a whole lot of
difference between C and C++ -- so the strategy of building a new Python
type around a C structure (pointer) type will also work for C++
objects.</p>
<p>For C++ libraries, you can look at <a class="reference external" href="http://www.riverbankcomputing.co.uk/sip/">SIP</a>, <a class="reference external" href="http://cxx.sourceforge.net/">CXX</a>, <a class="reference external" href="http://www.boost.org/libs/python/doc/index.html">Boost</a>, or <a class="reference external" href="http://www.scipy.org/site_content/weave">Weave</a>.
<a class="reference external" href="http://www.swig.org">SWIG</a> is a similar automated tool that only supports C libraries.</p>
</div>
<div class="section" id="i-added-a-module-using-the-setup-file-and-the-make-fails-why">
<h1><a class="toc-backref" href="#id18">12&nbsp;&nbsp;&nbsp;I added a module using the Setup file and the make fails; why?</a></h1>
<p>Setup must end in a newline, if there is no newline there, the build
process fails.  (Fixing this requires some ugly shell script hackery,
and this bug is so minor that it doesn't seem worth the effort.)</p>
</div>
<div class="section" id="how-do-i-debug-an-extension">
<h1><a class="toc-backref" href="#id19">13&nbsp;&nbsp;&nbsp;How do I debug an extension?</a></h1>
<p>When using GDB with dynamically loaded extensions, you can't set a
breakpoint in your extension until your extension is loaded.</p>
<p>In your <tt class="docutils literal">.gdbinit</tt> file (or interactively), add the command:</p>
<pre class="literal-block">
br _PyImport_LoadDynamicModule
</pre>
<p>Then, when you run GDB:</p>
<pre class="literal-block">
$ gdb /local/bin/python
gdb) run myscript.py
gdb) continue # repeat until your extension is loaded
gdb) finish   # so that your extension is loaded
gdb) br myfunction.c:50
gdb) continue
</pre>
</div>
<div class="section" id="i-want-to-compile-a-python-module-on-my-linux-system-but-some-files-are-missing-why">
<h1><a class="toc-backref" href="#id20">14&nbsp;&nbsp;&nbsp;I want to compile a Python module on my Linux system, but some files are missing. Why?</a></h1>
<p>Most packaged versions of Python don't include the
/usr/lib/python2.x/config/ directory, which contains various files required
for compiling Python extensions.</p>
<p>For Red Hat, install the python-devel RPM to get the necessary files.</p>
<p>For Debian, run <tt class="docutils literal"><span class="pre">apt-get</span> install <span class="pre">python-dev</span></tt>.</p>
</div>
<div class="section" id="what-does-systemerror-pyimport-fixupextension-module-yourmodule-not-loaded-mean">
<h1><a class="toc-backref" href="#id21">15&nbsp;&nbsp;&nbsp;What does &quot;SystemError: _PyImport_FixupExtension: module yourmodule not loaded&quot; mean?</a></h1>
<p>This means that you have created an extension module named &quot;yourmodule&quot;, but your module init function does not initialize with that name.</p>
<p>Every module init function will have a line similar to:</p>
<pre class="literal-block">
module = Py_InitModule(&quot;yourmodule&quot;, yourmodule_functions);
</pre>
<p>If the string passed to this function is not the same name as your
extenion module, the <tt class="docutils literal">SystemError</tt> exception will be raised.</p>
</div>
<div class="section" id="how-do-i-tell-incomplete-input-from-invalid-input">
<h1><a class="toc-backref" href="#id22">16&nbsp;&nbsp;&nbsp;How do I tell &quot;incomplete input&quot; from &quot;invalid input&quot;?</a></h1>
<p>Sometimes you want to emulate the Python interactive interpreter's
behavior, where it gives you a continuation prompt when the input
is incomplete (e.g. you typed the start of an &quot;if&quot; statement
or you didn't close your parentheses or triple string quotes),
but it gives you a syntax error message immediately when the input
is invalid.</p>
<p>In Python you can use the <tt class="docutils literal">codeop</tt> module, which approximates the
parser's behavior sufficiently.  IDLE uses this, for example.</p>
<p>The easiest way to do it in C is to call <tt class="docutils literal">PyRun_InteractiveLoop()</tt>
(perhaps in a separate thread) and let the Python interpreter handle
the input for you. You can also set the <tt class="docutils literal">PyOS_ReadlineFunctionPointer</tt>
to point at your custom input function. See <tt class="docutils literal">Modules/readline.c</tt> and
<tt class="docutils literal">Parser/myreadline.c</tt> for more hints.</p>
<p>However sometimes you have to run the embedded Python interpreter in
the same thread as your rest application and you can't allow the
<tt class="docutils literal">PyRun_InteractiveLoop()</tt> to stop while waiting for user input.  The
one solution then is to call <tt class="docutils literal">PyParser_ParseString()</tt> and test for
<tt class="docutils literal">e.error</tt> equal to <tt class="docutils literal">E_EOF</tt>, which means the input is incomplete).
Here's a sample code fragment, untested, inspired by code from Alex Farber:</p>
<pre class="literal-block">
#include &lt;Python.h&gt;
#include &lt;node.h&gt;
#include &lt;errcode.h&gt;
#include &lt;grammar.h&gt;
#include &lt;parsetok.h&gt;
#include &lt;compile.h&gt;

int testcomplete(char *code)
  /* code should end in \n */
  /* return -1 for error, 0 for incomplete, 1 for complete */
{
  node *n;
  perrdetail e;

  n = PyParser_ParseString(code, &amp;_PyParser_Grammar,
                           Py_file_input, &amp;e);
  if (n == NULL) {
    if (e.error == E_EOF)
      return 0;
    return -1;
  }

  PyNode_Free(n);
  return 1;
}
</pre>
<p>Another solution is trying to compile the received string with
<tt class="docutils literal">Py_CompileString()</tt>. If it compiles without errors, try to execute the returned
code object by calling <tt class="docutils literal">PyEval_EvalCode()</tt>. Otherwise save the input for
later. If the compilation fails, find out if it's an error or just
more input is required - by extracting the message string from the
exception tuple and comparing it to the string &quot;unexpected EOF while parsing&quot;.
Here is a complete example using the GNU readline library (you may
want to ignore SIGINT while calling readline()):</p>
<pre class="literal-block">
#include &lt;stdio.h&gt;
#include &lt;readline.h&gt;

#include &lt;Python.h&gt;
#include &lt;object.h&gt;
#include &lt;compile.h&gt;
#include &lt;eval.h&gt;

int main (int argc, char* argv[])
{
  int i, j, done = 0;                          /* lengths of line, code */
  char ps1[] = &quot;&gt;&gt;&gt; &quot;;
  char ps2[] = &quot;... &quot;;
  char *prompt = ps1;
  char *msg, *line, *code = NULL;
  PyObject *src, *glb, *loc;
  PyObject *exc, *val, *trb, *obj, *dum;

  Py_Initialize ();
  loc = PyDict_New ();
  glb = PyDict_New ();
  PyDict_SetItemString (glb, &quot;__builtins__&quot;, PyEval_GetBuiltins ());

  while (!done)
  {
    line = readline (prompt);

    if (NULL == line)                          /* CTRL-D pressed */
    {
      done = 1;
    }
    else
    {
      i = strlen (line);

      if (i &gt; 0)
        add_history (line);                    /* save non-empty lines */

      if (NULL == code)                        /* nothing in code yet */
        j = 0;
      else
        j = strlen (code);

      code = realloc (code, i + j + 2);
      if (NULL == code)                        /* out of memory */
        exit (1);

      if (0 == j)                              /* code was empty, so */
        code[0] = '\0';                        /* keep strncat happy */

      strncat (code, line, i);                 /* append line to code */
      code[i + j] = '\n';                      /* append '\n' to code */
      code[i + j + 1] = '\0';

      src = Py_CompileString (code, &quot;&lt;stdin&gt;&quot;, Py_single_input);

      if (NULL != src)                         /* compiled just fine - */
      {
        if (ps1  == prompt ||                  /* &quot;&gt;&gt;&gt; &quot; or */
            '\n' == code[i + j - 1])           /* &quot;... &quot; and double '\n' */
        {                                               /* so execute it */
          dum = PyEval_EvalCode ((PyCodeObject *)src, glb, loc);
          Py_XDECREF (dum);
          Py_XDECREF (src);
          free (code);
          code = NULL;
          if (PyErr_Occurred ())
            PyErr_Print ();
          prompt = ps1;
        }
      }                                        /* syntax error or E_EOF? */
      else if (PyErr_ExceptionMatches (PyExc_SyntaxError))
      {
        PyErr_Fetch (&amp;exc, &amp;val, &amp;trb);        /* clears exception! */

        if (PyArg_ParseTuple (val, &quot;sO&quot;, &amp;msg, &amp;obj) &amp;&amp;
            !strcmp (msg, &quot;unexpected EOF while parsing&quot;)) /* E_EOF */
        {
          Py_XDECREF (exc);
          Py_XDECREF (val);
          Py_XDECREF (trb);
          prompt = ps2;
        }
        else                                   /* some other syntax error */
        {
          PyErr_Restore (exc, val, trb);
          PyErr_Print ();
          free (code);
          code = NULL;
          prompt = ps1;
        }
      }
      else                                     /* some non-syntax error */
      {
        PyErr_Print ();
        free (code);
        code = NULL;
        prompt = ps1;
      }

      free (line);
    }
  }

  Py_XDECREF(glb);
  Py_XDECREF(loc);
  Py_Finalize();
  exit(0);
}
</pre>
</div>
<div class="section" id="how-do-i-find-undefined-g-symbols-builtin-new-or-pure-virtual">
<h1><a class="toc-backref" href="#id23">17&nbsp;&nbsp;&nbsp;How do I find undefined g++ symbols __builtin_new or __pure_virtual?</a></h1>
<p>To dynamically load g++ extension modules, you must recompile Python, relink it using g++ (change LINKCC in the python Modules Makefile), and link your extension module using g++ (e.g., &quot;g++ -shared -o mymodule.so mymodule.o&quot;).</p>
</div>
<div class="section" id="can-i-create-an-object-class-with-some-methods-implemented-in-c-and-others-in-python-e-g-through-inheritance">
<h1><a class="toc-backref" href="#id24">18&nbsp;&nbsp;&nbsp;Can I create an object class with some methods implemented in C and others in Python (e.g. through inheritance)?</a></h1>
<p>In Python 2.2, you can inherit from builtin classes such as int, list, dict, etc.</p>
<p>The Boost Python Library (BPL, <a class="reference external" href="http://www.boost.org/libs/python/doc/index.html">http://www.boost.org/libs/python/doc/index.html</a>)
provides a way of doing this from C++ (i.e. you can inherit from an
extension class written in C++ using the BPL).</p>
</div>
<div class="section" id="when-importing-module-x-why-do-i-get-undefined-symbol-pyunicodeucs2">
<h1><a class="toc-backref" href="#id25">19&nbsp;&nbsp;&nbsp;When importing module X, why do I get &quot;undefined symbol: PyUnicodeUCS2*&quot;?</a></h1>
<p>You are using a version of Python that uses a 4-byte representation
for Unicode characters, but some C extension module you are importing
was compiled using a Python that uses a 2-byte representation for
Unicode characters (the default).</p>
<p>If instead the name of the undefined symbol starts with
<tt class="docutils literal">PyUnicodeUCS4</tt>, the problem is the reverse: Python was built using
2-byte Unicode characters, and the extension module was compiled using
a Python with 4-byte Unicode characters.</p>
<p>This can easily occur when using pre-built extension packages.  RedHat
Linux 7.x, in particular, provided a &quot;python2&quot; binary that is compiled
with 4-byte Unicode.  This only causes the link failure if the extension
uses any of the <tt class="docutils literal"><span class="pre">PyUnicode_*()</span></tt> functions.  It is also a problem if an
extension uses any of the Unicode-related format specifiers for
<tt class="docutils literal">Py_BuildValue</tt> (or similar) or parameter specifications for
<tt class="docutils literal">PyArg_ParseTuple()</tt>.</p>
<p>You can check the size of the Unicode character a Python interpreter is
using by checking the value of sys.maxunicode:</p>
<pre class="literal-block">
&gt;&gt;&gt; import sys
&gt;&gt;&gt; if sys.maxunicode &gt; 65535:
...     print 'UCS4 build'
... else:
...     print 'UCS2 build'
</pre>
<p>The only way to solve this problem is to use extension modules compiled
with a Python binary built using the same size for Unicode characters.</p>
</div>
</div>
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